Leaking heater matrix...? No, just a weeping heater valve gasket... :)

I noticed some small splashes of water near the heater matrix, which alarmingly is close to the coil and the fuse box, luckily I caught it before it became a problem. On closer inspection, I could see coolant seeping between the heater matrix and the heater control valve which bolts onto it, thankfully then not a leaking heater matrix, time to remove it and see what's going...

On some cars tackling a leaky heater matrix means many hours spent removing the dashboard, moving wiring looms out of the way and various other components before you get to the matrix. On a DS the job is around 10 minutes, when you know what you're doing of course and thanks to Colin Smith of http://citroendsrestoration.blogspot.co.uk/ I was able to tackle the job with confidence.

Here's a couple of pointers that Colin helped me with that will help you too:

1. You do not have to drain the coolant, the two pipes on either side of the matrix are both high up and providing you tackle the job on a cold engine there will be no pressure or coolant in the pipes
2. Release the cable from the heater valve by undoing the clamp and the bolt on the valve.
3. All you now need to do to release the matrix from the heater box is to un-clip a spring clip which goes across the matrix.
4. You can then lift the matrix out, beware though, it will get stuck at the rear of the matrix since there is a copper coil that you need to ensure the top end of it is clear of the heater housing. I used a thin metal ruler which helped me push it back as I lifted.
5. Now that the matrix is on the bench, you will see a V shaped clip that is holding the heater valve coil element, carefully remove this to release the element.
6. Next, spray penetrating oil on the bolts holding the heater valve to the matrix, wait a few minutes and then tighten slightly to break any rust present. Then add more penetrating oil and unscrew slightly, add more penetrating oil, wait, and then unscrew again slowly until it comes off.
7. You will notice that there is a rubber o-ring that seals the heater valve with the matrix, replace.

Underneath the valve, showing O-ring

The culprit, brittle O-ring

With the matrix off it's a good idea now to flush it with water from the hose pipe until the water that runs through it is clean. Luckily mine was pretty clear which I gather is unusual, if yours isn't you may want to try a caustic solution or have it cleaned professionally.


With a new rubber O-ring sourced it was time to put it all together but importantly to replace a 10mm wide felt strip that surrounds the top and bottom of the matrix which over the years is usually very perished. The purpose of the felt is to provide a seal and thus contain the heat from the matrix within the plastic heater box and not allow it to escape which has the disadvantage of warming up the engine bay and not providing the full heat in the cabin for heating. I found that closed cell foam worked a treat here and even more conveniently I was able to purchase it in exactly the correct dimensions I required: 10mm width at 3mm thickness and 3metres length (more then enough) for £3.30 inclusive of delivery, the added bonus is it came with very sticky adhesive on one side. In hindsight 5-6mm thickness would be better but I was able to just double it up without issue.